Can a Pennsylvania landlord introduce a new fee mid-tenancy?

Generally no, on a fixed-term lease. The lease locks the rent and any specified fees for the term; introducing a new fee (parking, pet, trash, key replacement) without tenant agreement is a unilateral lease modification, not enforceable. On a month-to-month tenancy in Pennsylvania, the landlord can add a new fee with the same advance notice required for a rent increase (Pennsylvania uses 68 P.S. 250.501 tenure-based notice: 15 days for tenancies under 1 year, 30 days for 1 year or more. No state rent cap. Rent changes on month-to-month tenancies follow the same notice ladder.). Once the notice period passes, the tenant either accepts the new fee by continuing to pay or terminates the tenancy. Some fees are statutorily controlled regardless of the lease: late fees (no statewide cap), application fees, and fees that exceed the actual cost of the service. A "new" fee that effectively re-categorizes a previously-bundled cost is treated as a rent increase.

Source: 68 P.S. 250.501 (Pennsylvania Landlord and Tenant Act 1951)